Error if tags besides `gogm` contain built-in keywors

[nettrino]

In cases of structs containing relationship in their directives outside gogm, errors are encountered. For instance,

Region            *CloudRegion                    `json:"region,omitempty" custom:"rel=IN_REGION;dir=OUT" gogm:"relationship=IN_REGION;direction=outgoing"

works whereas

Region            *CloudRegion                    `json:"region,omitempty" custom:"relationship=IN_REGION;dir=OUT" gogm:"relationship=IN_REGION;direction=outgoing"

doesn't